1. Field of the Invention
The invention relates generally to a process for pretreating biomass material prior to conversion to liquid fuel, and more particularly to a pretreatment process comprising soaking biomass material in an aqueous solvent, which may sometimes also contain additive(s). In a preferred embodiment the pretreatment process comprises removal of inorganic materials from the biomass material.
2. Description of the Related Art
World energy demand is projected to increase substantially due to: an increase in the world's population; improvement of the standard of living in underdeveloped countries; and depletion of the reserves of fossil fuels.
Now, generally recognized by major countries, global climatic changes caused by increasing emissions of greenhouse gases, such as CO2, require that newly developed energy sources must be environmentally compatible and sustainable. Therefore, greener sources of energy are needed to replace or reduce the consumption of fossil fuels. Biomass is a sustainable and renewable source of fuel, with potentially a net zero greenhouse gas impact.
Biomass conversion technologies include: biological processes, such as anaerobic or aerobic fermentation; and thermal conversion processes, such as direct combustion for heating and generating electricity; gasification for producing syngas; and pyrolysis for producing biooils, which can be converted to fuels and chemicals.
The thermal conversion processes include hydrothermal processes, wherein biomass is treated in slurry form in autoclaves at temperatures above 200° C. and under autogenous pressures.
Pyrolysis processes have a high potential for large scale commercialization, as they provide flexibility in varying process conditions, such as heating rate, temperature, pressure, contact time, atmosphere, etc., to optimize yields of liquids (oil), gas and char. Of particular interest is fast (or flash) pyrolysis designed to convert the biomass to maximum amounts of oil, employing a very low residence time, a very high heating rate and temperatures up to 500° C. The oil produced has a high energy density that can be directly used in combustion or refined to fuels and specialty chemicals.
However, the pyrolysis-derived bio-oils, because of their high oxygen contents, high viscosity, corrosiveness and low stability, have limited direct applications as fuels. Intensive research is being now carried out to upgrade the quality of said bio-oils to products that are comparable to conventional fuels in composition, and chemical and physical properties.
Pretreatment processes of biomass before pyrolysis offer possible solutions in the form of biomass modification that will allow the pyrolysis process to be conducted at less severe conditions (i.e., lower temperatures, shorter contact times), and more efficiently in that more oil is produced, and the oil produces is of a better quality.
In an alternative approach, biomass conversion in large commercial plants is now carried out to produce ethanol, primarily using as feeds from renewable sources such as corn, sugar cane, and cereal grains. Because the cost of these raw materials represents roughly one-half of the total cost of the process to produce the ethanol, it is of paramount interest to use cheaper biomass raw materials for conversion to ethanol. Furthermore, it is important to utilize biomass sources other than grains, in order to minimize the impact on food prices.
Consequently, less costly lignocellulosic biomass materials derived from agricultural and forestry residues are very attractive for use as biomass sources to be converted to ethanol or other fuels.
The use of ethanol in automobile fuels not only reduces the need for petroleum (crude oil), but also substantially reduces the carbon dioxide car-exhaust emissions.
Commercial large scale operations involving the production of ethanol from cellulosic biomass use biological or non-biological processes to depolymerize (break down) the cellulose. The most commonly used biological processes use enzymes, whereas the nonbiological processes use an acid hydrolysis to convert the cellulose to sugars, mostly using dilute or concentrated sulphuric acid. These processes are considered as a pretreatment of the biomass in the overall bioconversion processes, which are followed by fermentation and distillation.
In the prior art one finds descriptions of other kinds of pretreatments such as steam explosion, which is followed by enzymatic hydrolysis, fermentation and distillation in the production of ethanol (see, C. E. Wyman et al, Bioresource Technology 96 (2005), 1959-1966).
Since the presently known processes for conversion of the lignocellulosic biomass (derived from agricultural and forestry residues) are more expensive than the processes used now commercially to produce ethanol from grains and cereals, there is strong interest in developing new or improved processes that will allow a more cost-effective and environmentally acceptable manner of converting lignocellulosic biomass (from residues derived from agriculture and forestry materials) to ethanol (see, N. Mosier et al, Bioresource Technology 96 (2005), 673-686).
In general, lignocellulosic biomass from such residues consists mainly of three components: cellulose, hemicellulose and lignin. The cellulose component is a polymer of glucose, formed in long strands. It is associated with the hemicellulose component layer, and both the crystalline cellulose and hemicellulose are encapsulated by the lignin cell wall.
In ethanol production from cellulosic biomass, the cellulose and hemicellulose are converted to sugars, such as glucose and xylose, followed by fermentation. Lignin is a 3-dimensional branched polyaromatic matrix acting as a sheath, or a protective coating to the cellulose and hemicellulose components of the biomass.
As a result, due to the differences in the bonding of the components, the high crystallinity of the cellulose, and the presence of the protective sheath of the lignin, the penetration of, and interaction with, the acids and/or the enzymes is highly impeded. This problem is much less observed when processing cereal grains to ethanol by converting the cellulose and hemicellulose to glucose, using acid or enzymatic hydrolysis.
However, for biomass from sources other than cereal grain, the lignin present resists the enzyme attack and hence lower yields are obtained. To at least partly overcome this problem, pretreatment of the biomass is necessary prior to subjecting the biomass to enzymatic hydrolysis. (see, T. A. Hsu, et al. Chemtech, May (1980), 315-319)
Since the major cost of the overall conversion process is due to the biomass feed and enzymes, it is necessary to minimize the use of enzymes and obtain the maximum conversion of the carbohydrates to ethanol.
For these reasons, a considerable amount of R&D work has been devoted during the last few years for developing means to pretreat the lignocellulosic biomass in such ways that the accessible surface area increases, which will allow an increase in the biomass ethanol conversion. The most popular processes are acid and enzymatic hydrolysis processes, which are used mostly to convert the cellulose and hemicellulose to glucose.
In the prior art there are several versions of the original acid hydrolysis process. These involve very concentrated acids or dilute acids, in one or two step treatments, and combinations of acid treatment with steam treatments, such as steam-explosion.
Overall, the pretreatment processes utilizing acids such as sulphuric acid, require specially constructed plant equipment that must be resistant to acid corrosion. Additionally, the use of acid requires neutralization by a low-cost base such as calcium hydroxide, and the formed sulphate salt must be filtered and washed from the biomass. This creates large waste streams that require disposal and lead to additional costs. Further, for the use of highly concentrated acids, the process requires an additional evaporator to produce/recycle the highly concentrated acid.
The acid pretreatments of lignocellulosic biomass feeds used to convert the cellulose and hemicellulose to fermentable sugars have certain disadvantages relating to higher costs, lower efficiencies and environmental problems. Specifically, processes using a high acid concentration have disadvantages relating to corrosion of equipment, and high cost waste stream disposal, whereas the dilute (low acid concentration) process produces a low conversion of the biomass to fermentable sugars.
Pretreatments using steaming (steam-explosion) as such and combinations with acid treatments have certain disadvantages. During steam-explosion pretreatments, the pentoses and hexoses produced from the hydrolysis of the cellulose material are to some extent further converted to undesirable by-products, such as furfural, levulinic acid (4-oxopentanoic acid) and formic acid, together with other products. (see, M. M. Wu et al, Appl. Biochemistry and Biotechnology 77 (1999) 47-54)
In general, processes involving acid treatments and steam-explosion produce compounds such as aliphatic acids, phenolic and furan derivatives. These degradation products act as inhibitors in subsequent processes using enzymes to convert the sugars to ethanol. (see, V. S. Chang, et al., Appl. Biochemistry and Biotechnology 84 (2000) 5-37)
Further, although high severity steam explosion allows the enzymes to react more effectively, it does degrade the produced sugars and reduces the yields, as well as making the lignin less reactive. Using less severe steaming-acid pretreatments produces lower glucose yields, since the enzymes cannot react with a major part of the cellulosic material. (see, J. Soderstrom, et al., Biomass and Bioenergy 24 (2003), 475-486—U.S. Pat. No. 4,880,473 and U.S. Pat. No. 6,692,578, U.S. Patent Applic. #US2005/0069998A1, U.S. Pat. No. 5,597,714, WO20061085762 A1)
Enzymatic hydrolysis presents a promising process for large-scale operations using lignocellulosic biomass, as it is low-energy intensive, environmentally compatible and does not require the use of corrosive chemicals. The main disadvantage of this process has been the cost of producing the enzymes, even though during the most recent years, with new improved processes, such cost has been reduced. (see, V. S. Chang et al., Applied Biochemistry and Biotechnology 84 (2000) 5-37)
Thus, there is need for developing pretreatment processes that allow the maximum conversion of lignocellulosic biomass to ethanol via high yield enzymatic hydrolysis, without the use of corrosive chemicals, waste streams and specialty high-cost equipment. There is a further need for such pretreatment processes that allow conversion of lignocellulosic biomass via pyrolysis process resulting in higher yields of bio-oil, as well as improved quality of bio-oil.
In general, any pretreatment of the lignocellulosic biomass to enhance its conversion must at least increase the micro- and macro-accessibility to the bulk of the particle allowing penetration of the enzymes and chemicals.
In the prior art, the term cellulases is used to describe a class of enzymes responsible for the biodegradation natural process. Cellulases are mainly produced by bacteria and fungi. For the purpose of this discussion, it is noted that the proteinic conveyors of the complex enzyme groups have molecular weights in the region of 30,000 to 100,000, and have globular shapes with hydrodynamic diameters in the range of 3 to 4 nm. Therefore, the openings of the cannulae, pores, cavities and interfibrillar interstices, must be large enough to allow molecules, ions, compounds, and enzymes to penetrate in the bulk of biomass. For an efficient enzymatic digestion and conversion, the biomass particle should have the largest possible number of such openings with diameters at least 3 to 4 nm. (see, H. A. Kzassig et al, in Polymer Monographs, “Cellulose”, vol. 11 (1993) p 202.)